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LT1308BIS8

High Current, Micropower Single Cell, 600kHz DC/DC Converters

Linear Technology 

LT1308A/LT1308B

APPLICATIONS INFORMATION

SHDN PIN

The LT1308A/LT1308B SHDN pin is improved over the

LT1308. The pin does not require tying to VIN to enable

the device, but needs only a logic level signal. The voltage

on the SHDN pin can vary from 1V to 10V independent

of VIN. Further, floating this pin has the same effect as

grounding, which is to shut the device down, reducing

current drain to 1μA or less.

LOW-BATTERY DETECTOR

The low-battery detector on the LT1308A/LT1308B fea-

tures improved accuracy and drive capability compared

to the LT1308. The 200mV reference has an accuracy of

± 2% and the open-collector output can sink 50μA. The

LT1308A/LT1308B low-battery detector is a simple PNP

input gain stage with an open-collector NPN output. The

negative input of the gain stage is tied internally to a 200mV

reference. The positive input is the LBI pin. Arrangement as

a low-battery detector is straightforward. Figure 10 details

hookup. R1 and R2 need only be low enough in value so

that the bias current of the LBI pin doesn’t cause large

errors. For R2, 100k is adequate. The 200mV reference

can also be accessed as shown in Figure 11.

R1

LBI

R2

100k

VIN

+

LT1308A

LT1308B

LBO

–

5V

100k

TO PROCESSOR

VBAT

200mV

INTERNAL

REFERENCE

GND

1308 F10

R1 = VLB – 200mV

2μA

A cross plot of the low-battery detector is shown in

Figure 12. The LBI pin is swept with an input which var-

ies from 195mV to 205mV, and LBO (with a 100k pull-up

resistor) is displayed.

VLBO

1V/DIV

195

200

205

VLBI (mV)

1308 F12

Figure 12. Low-Battery Detector

Input/Output Characteristic

START-UP

The LT1308A/LT1308B can start up into heavy loads, unlike

many CMOS DC/DC converters that derive operating voltage

from the output (a technique known as “bootstrapping”).

Figure 13 details start-up waveforms of Figure 1’s circuit

with a 20Ω load and VIN of 1.5V. Inductor current rises to

3.5A as the output capacitor is charged. After the output

reaches 5V, inductor current is about 1A. In Figure 14, the

load is 5Ω and input voltage is 3V. Output voltage reaches

5V in 500μs after the device is enabled. Figure 15 shows

start-up behavior of Figure 5’s SEPIC circuit, driven from a

9V input with a 10Ω load. The output reaches 5V in about

1ms after the device is enabled.

VOUT

2V/DIV

Figure 10. Setting Low-Battery Detector Trip Point

VBAT

200k

2N3906

VREF

200mV

+

10k

10μF

VIN

LBO

LT1308A

LBI LT1308B

GND

1308 F11

Figure 11. Accessing 200mV Reference

IL1

1A/DIV

VSHDN

5V/DIV

1ms/DIV

1308 F13

Figure 13. 5V Boost Converter of Figure 1.

Start-Up from 1.5V Input into 20Ω Load

1308abfb

11

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